Farhan and Chris,
With the GALI-6+ still in place (it's U2, after the first mixer), adding a 6 dB attenuator between the generator and Specan caused the distortion product to fall 17.9 dB.? But the tone level only dropped 0.8 dB.? I could see some harmonic distortion in the IF amplifier output with a scope, suggesting significant non-linearity in the signal chain.
Then completely removed the U2 GALI-6+ (and re-calibrated the firmware power meter constants).? There was already a 6 dB pad after the first mixer, so took the LO module output from that pad.? Results are shown below and are much improved.? With the generator directly connected to the Specan, the IIP3 is now -20 + 68/2 = 14 dBm (I think?).? When a 6 dB attenuator is inserted at the Specan input, the measured tones fall 5 dB and the distortion products fall below the noise floor.
This seems to vindicate the two-tone generator though don't have enough dynamic range for the 6 dB attenuator test.? This seems like usable performance, though I may search for a replacement for the MAV/GALI to improve the noise figure.
Thank you for your help,
--
David KV4FP
